Trump, Giuliani and other members of Trump team have claimed that the FBI's use of an informant inside the Trump campaign is evidence of a spying scandal.

Trump has openly accused the FBI of spying on his campaign -- even dubbing it "SpyGate" -- but intelligence officials say they were simply trying to probe Russia’s attempts to influence the 2016 election.

On "Outnumbered," Napolitano said that the use of an informant is standard operating procedure in intelligence gathering and in criminal investigations.

“[It] is such a stunningly unremarkable event, because law enforcement does this all the time," Napolitano said.

"Now, Rudy Giuliani said they put an undercover FBI agent in the campaign. There is zero evidence for that. That is such an outlandish and outrageous allegation. It should not have been made without showing any evidence," Napolitano said.

Watch the judge weigh in on the legitimacy of Special Counsel Robert Mueller's investigation below.